(1) Field of the Invention
The present invention relates to martensitic heat-resistant steel, in particular to a martensitic heat-resistant steel having an increased high temperature strength which is suitably used for turbine blades and the like.
(2) Description of the Prior Art
In existing steam turbines which operate at steam temperatures of up to 566.degree. C. and steam pressures of up to 246 atg, crucible steel 422 (12Cr-1Mo-1W-1/4V steel) or steel H46 (12Cr-Mo-Nb-V steel) is used for the blades and 1Cr-1Mo-1/4V steel or 11Cr-1Mo-V-Nb-N steel is used for the rotor shafts.
Recently, as the cost of fossil fuels such as petroleum and coal have been rising, it is important to improve the generator efficiency of thermoelectric power plants using such fossil fuels. It is necessary to raise the steam temperature or pressure of a steam turbine in order to increase the generator efficiency. Materials used for steam turbines have insufficient creep rupture strength and so stronger materials are needed.
Various kinds of materials having an increased high temperature strength have been proposed (for example, U.S. Pat. No. 3,139,337) and have been considerably effective. But these materials have insufficient creep rupture strength at temperatures higher than 550.degree. C.
In view of creep rupture strength, Ni-base alloys and Co-base alloys are superior but these materials are expensive in addition to having inferior workability and a low damping constant.